This is clearly not the case, well except for the "no studio is going to make a movie/TV show today that has a majority white cast" part.I don't think blackwashing is a thing?
Anyway, Amazon had little choice. If you want to make a show that has true global reach then you need to make it as diverse as possible, regardless if this goes against the cannon or not. Times have changed and no studio is going to make a movie/TV show today that has a majority white cast.
Does Wakanda in Wakanda Forever look very diverse to you? Is that film only marketed towards a black audience and, I suppose, an aquatic south american derived one? Obviously not. Was Squid Games diverse? Nope. What about Game of Thrones? Hell, the MASSIVELY popular Jackson LOTR films/Hobbit were cast to be more authentic to the books and that didn't seem to limit their reception.
A show can catch fire for any number of reasons, color wheel casting is primarily a marketing ploy and these days some sort of restitution effort if not a type of extortion. When awards shows start putting in arbitrarily made casting/hiring quotas then its only understandable that it will potentially reflect poorly on the final product. Where are the north africans in Wakanda? Where are the white south africans? If Wakanda is supposed to draw from across the continent, why does it exclude so many other groups? We know why, and we know why the same folks insisting on color wheel casting for a scandinavian lore inspired and "ancient northern europe" set show like LOTR are strangely quite about monoethnic casting in Wakanda or any other show not featuring white people. Anyone complaining about the casting in Mel Gibsons Apocalypto?
Folks will latch on to any possibility, however remote, of a person of a non-caucasian ethnicity getting into a historic european set project, fantasy or not, but won't mention a thing about asian, south american, african, etc etc stuff that are mono-ethnic.
For the record, I only cry out for consistency. I like Wakanda how it is currently depicted, I only ask that the same courtesy in representation is extended to other projects with just as much of a claim to "historic precedence" as the fantasy comic book country of Wakanda. It's either always ok, or never ok, which is it? If you don't want to see all white folk in your show, then STOP SETTING THEM IN A HISTORIC EUROPEAN BASED SETTING!!! Be original with the IP. Wheel of Time, with it's post-apocalypse America (??if that was ever confirmed) setting had a good claim to ethnic diversity, expect they blew it with the "remote tiny village isolated for centuries" Two Rivers that looked more like a large metropolitan trading center.